Method and apparatus for observing a low magnification electron microscope image

ABSTRACT

A method and apparatus for observing a low magnification wide field, low aberration electron microscope image characterized in that the objective lens is not used, the focal length of the final stage condenser lens is adjusted so as to focus a crossover image on an aperture located near an intermediate lens and the position of the specimen is kept fixed regardless of magnification range.

Bite States Patent [191 Yanaka et al.

[ Dec. 3, 1974 Nihon Denshi Kabushiki Kaisha, Tokyo, Japan 22 Filed: May 14, 1973 21 Appl. No.: 359,669

[73] Assignee:

{30] Foreign Application Priority Data May 22, 1972 Japan 47-50594 [52] US. Cl 250/311, 250/307 250/396 [51] Int. Cl. H0lj 37/26 [58] Field of Search 250/310, 311, 396, 398,

References Cited UNITED STATES PATENTS 2,494,442 1/1950 LePoole 250/396 2,802,110 8/1957 Kazato et al 250/311 3,090,864 5/1963 Takahashi et al... I 250/311 3,359,418 12/1967 Bahr et a1. 250/396 3,374,346 3/1968 Watanabe.... 250/396 3,560,781 2/1971 Riecke 250/396 3,629,575 12/1971 Emmasingelm. 250/396 3,715,582 2/1973 Akahori et a1 250/396 Primary ExaminerArchie R. Borchelt Assistant ExaminerB. C. Anderson Attorney, Agent, or Firm-Webb, Burden, Robinson & Webb 5 7 ABSTRACT A method and apparatus for observing a low magnification wide field, low aberration electron microscope image characterized in that the objective lens is not used, the focal length of the final stage condenser lens is adjusted so as to focus a crossover image on an aperture located near an intermediate lens and the position of the specimen is kept fixed regardless of magnification range.

5 Claims, 5 Drawing Figures FIRST connzusn LENS EXClTATluN CURRENT SvPPLY sum: CONDENSER LENS EXUTATION CURRENT SUPPLY 2i 0 DEFLELl'ING CURRENT SUPPLY OEJELTNE LENS ExLiTATiON CURRENT SUPPLY $T|CrMAToR LURRENT SUPPLY IMAGE mama Ll'NS iYSTM CURRENT svmt PATENTEUEEB 319M Y 3,852,597

saw 10? FIRST CONDENSER LENS EXClTATloN CURRENT SUPPLY SECOND CONDENSER LENS ExuTATwN CURRENT SQ LY DEFLEQTING' 25a, 25 :5 CURRENT SUPPLY omscnve LF NS EXCITATION CURRENT SUPPLY STKCXMATDR CURRENT SUPPLY IMAGE FoaMlN r LENS SYSTEM cmzazur suPPI-Y METHOD AND APPARATUS FOR OBSERVING A LOW MAGNIFICATION ELECTRON MICROSCOPE IMAGE This invention relates to a method and apparatus including an electron microscope for observing a low magnification image having high resolving power.

When observing a low magnification image with an electron microscope, the specimen is usually placed some little distance from the objective lens in order to increase the focal length and thereby widen the field of view, and also to enhance image contrast.

However, since the diameter of the objective lens aperture controls the field of view as well as the image contrast, the use of a small diameter objective lens aperture for enhancing image contrast has the disadvantage of restricting the field of view. For instance, if a micron diameter aperture is used when observing an image magnified 1000 times, the diameter of the image on the photographic film or fluorescent screen is only about 20mm. Even if a larger aperture is used at the sacrifice of contrast, the diameter of the image will only be 100mm at most, which corresponds to a field of view of about 100 microns. This is not sufficient for practical purposes. Again, if in order to enlarge the field of view, it is necessary to extend the focal length more than a specified amount, the specimen would have to be placed outside the objective lens pole gap which would cause the image forming electron beam to become prone to the effects of external magnetic field fluctuations. As'a result, image resolving power would be adversely affected.

Further, since a portion of the image forming electron beam passes through the peripheral area of the intermediate lens magnetic field in order to-enlarge the electron beam image, the image formed by the electron beam becomes distorted and blurred due to intermediate lens field (off-axial) aberration.

Thus, in utilizing the method according to the present state of the art, it is necessary to take several tens of photographs at a comparatively high magnification, for example 5,000 times, and join them together to make a composite picture of the image having the required field of view. However, due to inherent image distortion, it is difficult to fabricate a-sequentialcomposite picture. Another problem of consequence, especially when using vulnerable specimens, is the vulnerability of the specimen to irradiation damage in view of the extended irradiation time necessary to complete the photographing of a series of photographs. I

It should be pointed out inpassing that US. Pat. No.

. spond by preventing the specimen from moving during 3,629,575 describes a method for observing a low magnification image whereby the objective lens energizing current is adjusted to a fixed value lyingoutside the normal control range in order to shift the focal plane of the objective lens facing the image from the contrast intensifying diaphragm to the objective-limiting diaphragm. However, in this method, due to the weak objective lens excitation current, lens aberration is appreciable making it impossible to obtain a low magnification image having the required amount of resolving power.

It is, therefore, an object of this invention to provide a method and apparatus capable of displaying a high contrast, wide field, low aberration image.

Another object of this invention is to make the field of view of the low and high magnification images correimage observation.

These and other advantages of this invention will become apparent by reading the following description in conjunction with the accompanying drawings in which:

FIGS. 1 and 2 are schematic diagrams showing the electron microscope according to this invention;

FIG. 3 is a schematic diagram showing the electron beam path of the microscope optical system shown in FIG. 1 incorporating the method according to this invention;

FIG. 4 is a schematic diagram showing the electron optical system, complete with power circuits, of a practical embodiment of this invention; and,

FIG. 5 is a diagram showing the aberration features of electron optical system according to this invention.

Referring to FIG. 1, a microscope column 1 comprises a chamber 2 containing an electron gun 3, an anode 4, first and second condenser lenses 5 and 6, a specimen chamber 7 containing a specimen 8, an objective lens 9, first and second intermediate lenses l0 and 11, a projector lens 12 and a viewing chamber 13 complete with a viewing window 14 and a fluorescent screen 15. Excitation current supply sources 16 l7, l8, 19 are provided for the various lenses. A' switch 20 is arranged between the objective lens 9 and its excitation current supply source 18. A deflecting coil 21 is arranged above the sample and has an associated'deflecting current supply source 22. A stigmator coil 23 for the objective lens is associated with current supply source 24. A condenser lensaperture plate 25 is held by a supporting means 26. The aperture plate is provided with three apertures, 25a, 25b, and 250, as shown in FIG. 2. A contrast aperture plate 27 is held by a supporting means 28. a

FIG. 3 shows the electron beam path of the microscope optical system shown in FIG. '1 incorporating the method according to this invention. Although a four stage'image forming lens system is used in the embodiment as showmfive or more stages or three stages are, of course, applicable. Thespecimen 8'is placed in the magnetic flux gap between the pole pieces of the objective lens as in the-case of high magnification image observation. Although the incorporation of the method according to this invention does not necessarily means that the position of the specimen cannot be changed, it is, in the interests of low and high magnification image correspondence, to keep the specimen fixed. The aperture plate 27 is used for enhancing image contrastand can be used as a field limiting aperture in the conventional electron microscope.

In order to observe or photograph a low magnification wide field. image according to this invention, the objective lens current is first of all switched off. Next, the second condenser lens aperture plate ispositioned so that the largest aperture 25a having a diameter of 0.5mm or more aligns with the beam path and the contrast aperture plate 27 is similarly inserted in the electron beam path by means of the supporting means 28. The axis of the electron irradiating lens system including the final stage condenser lens 6 and the axis of the first intermediate lens 10 are then mutually aligned by means of the deflecting coil. The condenser lens excitation current is adjusted so as to form a crossover image on the aperture plate 27. Finally, the first intermediate lens current is adjusted so as to produce a specimen imageon the image source point of the second interme diate lens 11. By so doing, a low magnification wide field image is displayed on the fluorescent screen 15.

FIG. 4 shows the electron optical system, complete with power circuits, of a practical embodiment utilizing the method according to this invention. In this embodiment, the supply source 22 of the deflecting coil 21 is controlled by the switch. 20. When the switch is disengaged, the axis of the electron beam irradiating lens system is automatically aligned with the axis of the first flecting coil 21. The current supply source 17, which supplies excitationcurrentto the condenser lens 6 so as to form a crossover point image on the aperture plate 27 when the switch 20 is disengaged, is interlocked with the switch 20. Incidentally, by interlocking the switch 20 with the image forming lens system current supply source 19 which controls an image: magnification, it is possible to facilitate the observation of a low magnification image utilizing. the method according to this invention. In this case, the switch 20 is disengaged automatically when .the image magnification decreases to below, let's say 1,000 times (or 2,000 times or3,000 times if desired) by controlling the current supply source 19. 7

The field of view of the low magnification image thus obtained is determined by the following equation,

where a is the diameter of the observed specimen area, b is the diameter of the aperture 25, K is the distance between the aperture and the specimen 8 and L is the distance between the specimen and the contrast aperture 27. Since K=L, a is roughly b/2. Thus, by utilizing a 0.5 mm diameter condenser lens aperture, a field of view of approximately 250 microns is obtained, a great improvement on the I00 micron field of view obtained with the conventional method.

, Referring to image contrast, the intensity of the contrast depends on the aperture angle oz, at which the electrons transmitted through the specimen pass through the contrast aperture 27 and the quantity of electrons scattered by the specimen which are blocked by the aperture plate 27, as shown by the broken lines in FIG. 3. The aperture angle or is given by a, C/L, where C is the diameter of the aperture 27. Thus, if C is 20 microns, the aperture angle ct will be 10 rad. or smaller depending on the value of L which is roughly 100 mm' to 300 mm in conventional electron microscopes. This compares ver y favorably with the aperture angle utilizing the method as practiced by the present state of the art which is about 10 rad.; at best l0 rad. assuming a very small diameter contrast aperture 1 is used.

the aperture angle is 10 rad. or smaller, due to its negligible proportions.

Image blurring caused by image field curvature and field astigmatism is also proportional to ct and, since the electron beam passes near the optical axis of the first intermediate lens, the various aberration coefficients are minimized.

Now, under the condition that the electron beam having an angle a with respect to the optical axis passes at a distance X from the optical axis, through the specimen, the blurring factor AF due to field chromatic aberration and the over fringe width Dsf due to the defocus factor caused by image field curvature and field astigmatism are given as follows based on Glasers Blenden Freien System" of calculation.

where 7t is the wavelength of the electron beam, it and K are parameters, Ba, Ca, Da, Fa, Cv, Cf are the various aberration coefficients and V and AV are the electron accelerating voltage and its fluctuation factor respectively. Here, the angle a, is given by a X,,/l by utilizing the distance 1 between the specimen and the crossover point image.

FIG. 5 shows the values of AF and Dsfwith respect to I. In the figure, A indicates the position I L at which the contrast aperture plate is placed. Under the condition that the crossover pointimage is formed on said aperture, Dsf and AF are very small.

Finally, the axial astigmatism of the first intermediate lens is adequately corrected by the objective lens stigmator 23 without the need for fitting an additional intermediate lens stigmator. I

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

We claim:

l. A method for observing a low magnification image using a typical electron microscope having an electron beam condenser lens system including one or more lenses, and a projector lens'system including an objective lens and one or more intermediate lenses arranged in sequential order along the optical axis of the microscope, an imaging means and means for adjusting the excitation current to the lens comprising the steps for:

A. keeping the specimen arranged between the objective lens magnetic gap fixed;

B. disconnecting the objective lens excitation current; I

C. replacingthe final stage condenser lens aperture by an aperture having a diameter of at least 0.5mm;

D. inserting a contrast aperture in the electron beam path adjacent the intermediate lens on the object side thereof;

E. adjusting the current in the condenser lens system so as to form a focal point at the contrast aperture; and,

F. adjusting thecurrent in the projector lens system to focus a low magnification image on the imaging means.

2. In an electron microscope incorporating:

A. a condenser lens system including at least one condenser lens and associated excitation current sources;

B. a projector lens system including an objective lens and at least intermediate lenses and associated excitation current sources;

C. an imaging means;

the improvement comprising:

D. a switching means arranged between the objective lens and its associated excitation current supply source for disconnecting the excitation current;

E. means for replacing the final stage condenser lens aperture with an aperture having a diameter at least 0.5mm;

F. means for placing a contrast aperture in the electron beam path adjacent the intermediate lens on the object side thereof;

G. first means for adjusting the condenser lens excitation current soas to form a focal point on the contrast aperture when observing a low magnification image; and,

H. second means for adjusting the current in the projector lens system to focus a low magnification image on the imaging means.

3. An electron microscope as described in claim 2 characterized in that said first means for adjusting the condenser lens excitation current is interlocked with said switching means.

4. In an electron microscope incorporating:

A. a condenser lens system including at least one condenser lens and associated excitation current sources;

B. a projector lens system including an objective lens and at least one intermediate lens and associated excitation current sources;

C. an imaging means;

the improvement comprising:

D. a switching means arranged between the objective lens and its associated excitation current supply source for disconnecting the excitation current;

E. means for replacing the final stage condenser lens aperture with an aperture having a diameter of at least 0.5mm;

F. means for placing a contrast aperture in the electron path adjacent the intermediate lens on the object side thereof;

G. first means for adjusting the condenser lens excitation current so as to form a focal point on the contrast aperture when observing a low magnification image;

H. a deflecting means located below the final stage condenser lens, the excitation current source supply of said deflecting means being interlocked with said switching means so as to align the electron beam' with the optical axis of the intermediate lens; and,

I. second means for adjusting the current in the projector lens system to focus a low magnification image on the imaging means.

5. In an electron microscope incorporating:

A. a condenser lens system including at least one condenser lens;

B. a projector lens system including an objective lens and at least one intermediate lens;

C. an imaging means;

the improvement comprising:

D. a switching means arranged between the objective lens and its associated excitation current supply source for disconnecting the excitation current;

E. means for replacing the final stage condenser lens aperture with an aperture having a diameter of at least 0.5mm;

F. means for placing a contrast aperture in the electron beam path adjacent the intermediate lens on the object side thereof;

G. first means for adjusting the condenser lens excitation current so as to form a focal point on the contrast aperture when observing a low magnification image; and,

H. second means for adjusting the current in the projector lens system to focus a low magnification image on the imaging means, said second means being interlocked with said switching means so as to cut off the objective lens excitation current when observing a low magnification image.

' amiss EATEN'E" @FFRIE (IERTHF QAiE QQRREQTMN Patent No. 3, 852, 597 Dat d December 3, 1974 inventor-(S) Takashi Yanaka and Kohei Shirota It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 Line 51, should read --0 sign should be deleted Signed and sealed this 18th day of February 1975.

(SEAL) Attest 2 C, MARSHALL DANN RUTH Ca MASON Commissioner of Patents Attesting Officer and Trademarks FORM PO-IQSO (10-69) USCOMM-DC 5O376-P69 uas. GOVERNMENT PRINTING OFFICE: 1969 o-ass-su 

1. A method for observing a low magnification image using a typical electron microscope having an electron beam condenser lens system including one or more lenses, and a projector lens system including an objective lens and one or more intermediate lenses arranged in sequential order along the optical axis of the microscope, an imaging means and means for adjusting the excitation current to the lens comprising the steps for: A. keeping the specimen arranged between the objective lens magnetic gap fixed; B. disconnecting the objective lens excitation current; C. replacing the final stage condenser lens aperture by an aperture having a diameter of at least 0.5mm; D. inserting a contrast aperture in the electron beam path adjacent the intermediate lens on the object side thereof; E. adjusting the current in the condenser lens system so as to form a focal point at the contrast aperture; and, F. adjusting the current in the projector lens system to focus a low magnification image on the imaging means.
 2. In an electron microscope incorporating: A. a condenser lens system including at least one condenser lens and associated excitation current sources; B. a projector lens system including an objective lens and at least intermediate lenses and associated excitation current sources; C. an imaging means; the improvement comprising: D. a switching means arranged between the objective lens and its associated excitation current supply source for disconnecting the excitation current; E. means for replacing the final stage condenser lens aperture with an aperture having a diameter at least 0.5mm; F. means for placing a contrast aperture in the electron beam path adjacent the intermediate lens on the object side thereof; G. first means for adjusting the condenser lens excitation current so as to form a focal point on the contrast aperture when observing a low magnification image; and, H. second means for adjusting the current in the projector lens system to focus a low magnification image on the imaging means.
 3. An Electron microscope as described in claim 2 characterized in that said first means for adjusting the condenser lens excitation current is interlocked with said switching means.
 4. In an electron microscope incorporating: A. a condenser lens system including at least one condenser lens and associated excitation current sources; B. a projector lens system including an objective lens and at least one intermediate lens and associated excitation current sources; C. an imaging means; the improvement comprising: D. a switching means arranged between the objective lens and its associated excitation current supply source for disconnecting the excitation current; E. means for replacing the final stage condenser lens aperture with an aperture having a diameter of at least 0.5mm; F. means for placing a contrast aperture in the electron path adjacent the intermediate lens on the object side thereof; G. first means for adjusting the condenser lens excitation current so as to form a focal point on the contrast aperture when observing a low magnification image; H. a deflecting means located below the final stage condenser lens, the excitation current source supply of said deflecting means being interlocked with said switching means so as to align the electron beam with the optical axis of the intermediate lens; and, I. second means for adjusting the current in the projector lens system to focus a low magnification image on the imaging means.
 5. In an electron microscope incorporating: A. a condenser lens system including at least one condenser lens; B. a projector lens system including an objective lens and at least one intermediate lens; C. an imaging means; the improvement comprising: D. a switching means arranged between the objective lens and its associated excitation current supply source for disconnecting the excitation current; E. means for replacing the final stage condenser lens aperture with an aperture having a diameter of at least 0.5mm; F. means for placing a contrast aperture in the electron beam path adjacent the intermediate lens on the object side thereof; G. first means for adjusting the condenser lens excitation current so as to form a focal point on the contrast aperture when observing a low magnification image; and, H. second means for adjusting the current in the projector lens system to focus a low magnification image on the imaging means, said second means being interlocked with said switching means so as to cut off the objective lens excitation current when observing a low magnification image. 